1. Field of the Invention
The present invention relates to a webbing retractor which structures a seat belt device of a vehicle or the like.
2. Description of the Related Art
A seat belt device which restrains, by an elongated, strip-shaped webbing belt, the body of a vehicle occupant who is seated in a seat of a vehicle, is equipped with a webbing retractor which is fixed to the vehicle body at the side of the seat. The webbing retractor has a spool (take-up shaft) whose axial direction runs, for example, substantially along the longitudinal direction of the vehicle. The proximal end side, in the longitudinal direction, of the webbing belt is anchored at the spool. The spool accommodates the webbing belt in a state in which the webbing belt is wound the form of a roll around the outer peripheral portion of the spool.
An urging member, such as a tension coil spring or the like which urges the spool in a take-up direction in which the spool takes up the webbing belt, is provided at the webbing retractor. Due to the urging force of this urging member, the webbing belt is taken up and accommodated. In the state in which the webbing belt is applied to the body of a vehicle occupant, slack or the like of the webbing belt is eliminated by the urging force of the urging member. In this type of webbing retractor, the webbing belt, which has been taken-up on the spool, is pulled out by a tongue plate, which is provided at the longitudinal direction intermediate portion of the webbing belt, being pulled against the urging force of the urging member. In this state, the webbing belt can be applied to a vehicle occupant by making the tongue plate be held at a buckle device provided at the side of the seat opposite the side at which the webbing retractor is provided.
On the other hand, in order to even more strongly restrain the body of a vehicle occupant at the time of a rapid deceleration of the vehicle or the like, a structure has been conceived of in which, at the time of a rapid deceleration of the vehicle or the like for example, the driving force of a motor is applied to the spool. Due to this driving force, the spool is rotated by a given amount in the direction of taking the webbing belt up, such that the tension of the webbing belt increases.
In this type of webbing retractor, when the spool and the driving shaft of the motor are mechanically connected at usual times, this impedes rotation of the spool during usual taking-up and pulling-out of the webbing belt, and an unnecessary load is applied to the motor. Thus, a clutch mechanism is provided between the driving shaft and the spool.
Various types of clutch mechanisms can be thought of. As one example, there is a centrifugal clutch mechanism having a driving-side gear which is directly or indirectly connected to the driving shaft; a spool-side gear which meshes with teeth formed at the spool; and a plurality of pawls which are pivotally supported at the driving-side gear so as to be able to swing at the radial direction side of the spool-side gear which is eccentric with respect to the axial center of the driving-side gear, and so as to be able to mesh with the spool-side gear due to this swinging.
In this type of centrifugal clutch mechanism, when the plurality of pawls swing due to centrifugal force at the time when the driving-side gear rotates due to driving force of the motor, the pawls mesh with the spool-side gear. The driving shaft of the motor and the spool are thereby mechanically connected.
In the above-described centrifugal clutch mechanism, in order for the driving shaft of the motor and the spool to be mechanically connected, the pawls must mesh with the spool-side gear. However, the pawls do not just merely swing, but also, must rotate together with the driving-side gear in order to receive centrifugal force. Accordingly, depending on the positions of the pawls at the time of swinging, the pawls do not always enter into the tooth spaces of the spool-side gear. For example, there is a possibility that the pawls will abut the addenda of the spool-side gear at the time of swinging.
In this way, in the state in which the pawls abut the addenda of the spool-side gear, the driving shaft of the motor and the spool are not mechanically connected until the pawls enter into the tooth spaces due to further rotation of the driving-side gear. Thus, in a case in which the pawls abut the addenda, time is required from the start of driving of the motor to the time when the spool is actually rotated. Therefore, there is still room for improvement.